The present invention relates to silicon rubber compositions, and more particularly, the present invention relates to silicone rubber compositions which are suitable for liquid injection molding machines.
One type of composition that forms silicone elastomers is a SiH-olefin platinum catalyzed composition. Such compositions comprise as their basic ingredients a vinyl-containing polysiloxane, a hydride cross-linking agent which may be a hydride resin or a linear hydride-containing polysiloxane and a platinum catalyst. There are various types of platinum catalysts that can be utilized and particularly the solubilized platinum complex catalysts. The composition is packaged such that the hydride is kept separate from the vinyl polymer. The platinum is packaged with the vinyl polymer. Since there is no single package in which all three ingredients are present, the composition is stable. However, when all three ingredients are mixed, and in the absence of an inhibitor, the composition cures to a silicone elastomer at room temperature. Such compositions are for instance disclosed in Modic U.S. Pat. No. 3,436,366, which is hereby incorporated by reference.
There may be incorporated into such composition various types of inhibitors such as hydroperoxide-containing compounds, alkenyl maleates, allylisocyanurates, acetylenic alcohols and other types of inhibitors. These inhibitors generally allow the composition to have an appropriate or desirable work life and pot life at room temperature. However, when the composition is heated at elevated temperatures, that is, temperatures above 100.degree. C., the inhibitor decomposes and the composition cures in a matter of minutes or even in a matter of seconds to a silicone elastomer. In some cases, it may be desirable to post-bake the composition so as to complete the cure. Such compositions are also known in which the basic components contain fluorine substitution as disclosed in Jeram U.S. Pat. Nos. 4,041,010 and 4,029,629 which are hereby incorporated by reference. The advantages of such fluoro-substituted silicone compositions is that the composition has good solvent resistance.
These compositions disclosed above may contain various types of fillers including reinforcing fillers such as fumed silica and precipitated silica and also extending fillers such as lithopone, zinc oxide, diatomaceous earth, glass fibers, etc.
Proceeding to a related subject, there were developed injection moldimg machines for the fabrication of small plastic parts at a rapid rate. These liquid injection molding machines were capable of producing a large number of platic parts in a very small time period, and substantially eliminating much of the labor involved in the manufacture of such parts. As a result, the cost of the production of such plastic parts dramatically decreased. In accordance with this development of liquid injection molding machines, it became desirable to develop silicones for utilization in such liquid injection molding machines. An example and modification of liquid injection molding machines both of the plunger or ram type and of the reciprocating screw type is to be for instance found in the disclosures of Laghi, Ser. Nos. 159,262, 183,620. These patent applications disclose modifications that can be made to standard liquid injection molding machines such that they can handle liquid silicone compositions. Accordingly, such machines would take liquid silicone, force it into the barrel of the liquid injection molding machine under pressure, and then the machine would force a shot or volume of silicone into a mold and the mold would be heated. Upon heating, the silicone composition in the mold would cure to a silicone elastomer in a matter of seconds. The mold would then open and the part was automatically ejected. It was found that the most suitable silicone compositions for such liquid injection molding machines were the SiH-olefin platinum catalyzed type as described above, and particularly types that were developed to meet the requirements of the liquid injection molding industry. Accordingly, one type of part for which silicone compositions were desirable were electrical connectors. For such electrical connectors it was desirable that the cured silicone part had certain properties. Thus, it was desirable the cured elastomer have a Tensile Strength in the neighborhood of 800 to 1000 psi, an Elongation above 200, and more preferably in the range of 200-400%, a Tear above 100 pi and a Durometer Shore A Hardness above 60. Especially desirable in the case of electrical connectors was that the Durometer of the cured composition be above 60 and there be an Elongation of at least 200% prior to post-bake.
Accordingly, such a composition was formulated for liquid injection molding machines to make certain types of electrical connectors. It was also desirable in such a composition that the uncured composition have a viscosity that was not above 1.5.times.10.sup.6 centipoise at 25.degree. C. When the viscosity of the uncured composition was above this point, then the composition could not be injected or utilized in the liquid injection molding machine with facility; that is, the high viscosity composition would flow with difficulty into the mold, and a good molded part would not always be formed. Accordingly, such compositions were formulated in which there was obtained the foregoing Elongation and Durometer Hardness by using fumed silica as a reinforcing filler and ground quartz as the extending filler. The fumed silica imparts Hardness and increases the Tensile Strength, Elongation and Tear Strength while ground quartz imparts to the composition hardness. Further, the ground quartz was an inexpensive filler. However, it was found that such a filler had poor wearing effects on the mold. It resulted in an electrical connector that was abrasive. As a result, the abrasive silicone composition would wear out the mold such that after about two weeks continuous running or so of the mold, the liquid silicone composition would wear out a portion of the mold.
Accordingly, it became highly desirable to find a substitute for ground quartz. However, the insertion of plasticizers into the composition was not desirable; that is, traditional plasticizers that had been inserted into silicone compositions for maintaining the elongation of the composition, while at the same time reducing the abrading properties of the cured silicone composition were not found to be acceptable in the instant case. Such plasticizers such as linear diorganopolysiloxane fluids would decrease the Durometer of the cured composition which was not acceptable. Accordingly, it became highly desirable to find an ingredient or a filler for SiH-olefin platinum catalyzed compositions which could be substituted for the ground quartz and would not deleteriously affect the desired physical properties of the composition, that is, Durometer, Elongation, Tensile Strength, and Tear.
Accordingly, it was highly unexpected to find that calcined diatomaceous earth in certain concentrations in combination with fumed silica would give a desirable cured silicone elastomeric product in a liquid injection molding machine.
Accordingly, it is one object of the present invention to provide for a combination of fumed silica and diatomaceous earth in a liquid injection molding composition wherein the cured part has a Durometer Hardness of at least 60.
It is an additional object of the present invention to provide for an SiH-olefin platinum catalyzed composition that has a Durometer of at least 60, an Elongation of at least 200%, and which composition is not abrasive.
It is still an additional object of the present invention to provide for a liquid injection molding composition which has low abrasive properties in the cured state and good mold release properties and has acceptable physical properties for use in electrical connectors.
It is yet an additional object of the present invention to provide a process for operating a liquid injection molding machine of either the plunger type or the reciprocating screw type with a liquid silicone composition which cures to a silicone elastomer with the required physical properties.
It is still a further object of the present invention to provide a process for producing a cured part from a liquid silicone composition in a liquid injection molding machine wherein the silicone composition does not abrade the mold and wherein the cured silicone part has the desired physical properties such as a Durometer Hardness of at least 60 and an Elongation of at least 200%.
These and other objects of the present invention are accomplished by the disclosures set forth herein below.